1. Field of the Invention
The present invention relates to hard disk drives. More particularly, it relates to a disk drive spindle motor configured to reduce thermal expansion induced disk slip.
2. Description of the Prior Art and Related Information
A huge market exists for mass-market host computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive must be relatively inexpensive, and must accordingly embody a design that is adapted for low-cost mass production. Numerous manufacturers compete in this huge market and collectively conduct substantial research and development, at great annual cost, to design and develop innovative hard disk drives to meet increasingly demanding customer requirements.
Each of numerous contemporary mass-market hard disk drive models provides relatively large capacity, often in excess of 1 gigabyte per drive. Nevertheless, there exists substantial competitive pressure to develop mass-market hard disk drives that have even higher capacities and that provide rapid access. Another requirement to be competitive in this market is that the hard disk drive must conform to a selected standard exterior size and shape often referred to as a "form factor." Generally, capacity is desirably increased without increasing the form factor or the form factor is reduced without decreasing capacity.
Satisfying these competing constraints of low-cost, small size, high capacity, and rapid access requires innovation in each of numerous components or subassemblies. Typically, the main subassemblies of a hard disk drive are a head disk assembly and a printed circuit board assembly.
The head disk assembly includes an enclosure including a base and a cover; at least one disk having at least one recording surface; a spindle motor causing each disk to rotate; and an actuator arrangement. The actuator arrangement includes a separate transducer for each recording surface, and is moveable to position each transducer relative to the recording surface. The printed circuit board assembly includes circuitry for processing signals and controlling
operation of the drive. Improvements in spindle motor design, and in particular the relationship between the spindle motor and the rotating disk(s), have given rise to increased performance capabilities of the disk drive. For example, by centrally positioning the disk(s) directly to the spindle motor allows for the use of multiple disks, thereby increasing overall storage capacity. While this advancement has provided highly beneficial results, inherent characteristics of the spindle motor components and the disk media have also created certain design problems, as described below.
A disk drive spindle motor typically includes a central shaft, a bearing assembly, a stator, a magnet and a rotor ("hub"). The bearing assembly allows the hub to rotate about the shaft and the hub is concentrically positioned about the shaft. The magnet is normally divided into two or more arcuate sections, each of which is connected to an interior surface of the hub. Finally, the stator includes a series of coils and is concentrically positioned about the shaft, adjacent the magnet. With this general configuration, the various coils of stator are selectively energized to form an electromagnet that pulls/pushes on the rotor magnet sections, imparting a rotational motion onto the hub. Notably, the bearing assembly can assume a wide variety of forms, such as an upper and lower bearing portion, and locations and the disk(s) are typically directly secured to a flange portion of the hub. With this arrangement, rotation of the hub results in rotation of the attached disk(s).
The above-described spindle motor design may have certain problems due to the particular materials used for various components. As a starting point, it should be recognized that the disk(s) is typically made from aluminum. Conversely, at least some of the spindle motor components must be formed from steel (including magnetic steel). Steel and aluminum have different thermal coefficients of expansion. Because the disk drive, including the spindle motor and disks, is normally subjected to a wide temperature variation during manufacture subsequent storage and operation in a host computer, the resulting difference in thermal expansion may result in what is commonly referred to as "disk slip." Where, for example, the hub is made of steel, the aluminum-based disk(s) may "slip" relative to the hub flange when the disk drive is subjected to drastic variations in temperature.
In order to address potential disk slip, spindle motors incorporating a hub made of aluminum material have been envisioned. With this approach, it is believed that the hub and disk(s) will contract/expand in a virtually identical fashion so that the disk(s) will not slip relative to the hub flange. Importantly, where an aluminum hub is employed, an additional magnetic steel back iron must also be included with the spindle motor design to complete the magnetic circuit with the stator. The magnetic steel back iron is attached to the aluminum hub, between an inner surface of the hub and the separate magnet component.
While use of the aluminum hub may reduce disk slip resulting from expansion differences between the hub and the disk(s), other concerns arise. More particularly, the back iron is made of steel, having a thermal coefficient of expansion less than that of the aluminum hub, and is directly secured to the hub. Therefore, when the disk drive is subjected to drastic temperature variations, the aluminum hub will attempt to contract or expand at a greater rate than the steel back iron. The back iron impedes this natural contraction. However, an aluminum disk abutting the hub flange will contract and once the friction between the contact surface of the hub flange and the disk is overcome by the disk, disk slip may occur.
U.S. Pat. No. 4,717,977 discloses one way of solving problems resulting from different thermal coefficient of expansion rates for an aluminum sleeve ("hub") and an outer steel sleeve ("back iron") in a spindle motor of a disk drive. A magnet is attached to the outer steel sleeve in which the axial length of the outer steel sleeve is substantially greater than the axial length of the magnet. The problem is solved by having two clearances (`gaps") between the two sleeves. The two sleeves are secured together near the center of their overlapping areas. However, a spindle motor using such an approach is difficult to assemble which increases the costs of manufacturing the spindle motor.
Accordingly, substantial research and development efforts have been in expended to provide an improved spindle motor design that reduces thermal expansion induced disk slip while minimizing the manufacturing costs.